Instead, it is the bottom of Mike Napoli's left foot that poses the greatest threat to his ability to play regularly in the season's final 32 games, and by extension, be a force in the middle of the Boston Red Sox's lineup as they try to hold off the Tampa Bay Rays in the final weeks of the season.

Napoli is suffering the effects of plantar fasciitis, an odd-sounding condition whose name, like the innocuous-sounding "concussion," offers no clue to how debilitating it can be.

The plantar fascia is a thick fibrous band of tissue that starts at the heel bone and extends along the sole of the foot toward the toes. When it becomes inflamed, it can become extremely painful.

Napoli said the condition first flared on him when the team was in Anaheim the weekend of July 5-7.

"I feel like I can be in there every day," he said. "I thought that before and I was doing it. Some days are better than others but I think I'm pretty tough to be able to grind through some stuff. I'm just going to go one day at a time."

Until Saturday, Napoli had not been in the starting lineup since he aggravated the condition sliding into second on a double against the Yankees on Aug. 16. He has made just three plate appearances since, entering in the eighth inning of two games against the Giants in San Francisco last week.

He returned to the lineup Saturday, playing first base and batting cleanup against Dodgers left-hander Hyun-Jin Ryu, after taking a cortisone injection in his foot. He was 3-for-4 on the afternoon and singled in a run and then scored on Jonny Gomes' three-run homer in the first inning of Boston's 4-2 victory over the Dodgers.

"I got to play a couple of innings here and there the past week, and it's felt all right," he said Saturday morning. "The reason why I got the injection was because I was at a point where it was bothering me pretty good. I'm going to go into treatment for every day, like I've been, and try to take care of it as best as I possibly can.

"I'm just going to have to grind through it and play through some pain -- which I have been already."

It's an open question, though, of how long Napoli will be able to play by simply willing himself to do so. Earlier this week, the Los Angeles Angels shut down their star slugger, first baseman Albert Pujols, for the rest of the season after he tried playing through the condition.

"Everyone is different," Napoli said when asked if he found the Pujols situation worrisome. "Everyone can handle things differently. I don't know what was going on in his situation. Only time will tell. We'll see what happens. I'm going to try to stay out on the field and deal with it as much as possible."

Napoli insisted the condition has not affected him at the plate.

"I don't feel it one bit hitting," the Red Sox first baseman said. "That has nothing to do with anything. Running is definitely the worst part of it -- sometimes in the field, going to first base."

Manager John Farrell admitted to being uncertain about how much Napoli will be able to play the rest of the way, noting that there will be times he'll probably have to use Mike Carp or Daniel Nava at first base.

He also acknowledged that Will Middlebrooks took some ground balls at first during the last homestand, but with Middlebrooks playing so well at third, Farrell said he was reluctant to make a change. He did not rule it out, however, and with 20-year-old Xander Bogaerts available, that option would appear likely to be on the table if Napoli's condition worsens.